1. Field of the Invention
This invention relates to a high resolution scanner, and more particularly, to such a scanner which is particularly suitable for scanning photographic film.
2. Description of the Prior Art
In certain types of scanners for digitally recording an image from photographic film, the film is advanced relative to an illumination source which is adapted to provide a line of illumination on the film. Light transmitted through the film is imaged onto a photodetector such as a charge-coupled device (CCD). One of the main problems in such scanners is in projecting an image without aberrations on the photodetector so that the recorded image will be a true representation of the image on the film.
U.S. Pat. No. 4,467,361, is directed to one approach for producing an aberration-free image on an image sensor. This patent discloses an image pick-up apparatus which includes a solid state image sensor having an number of image sensing elements and an objective lens for projecting an image of the object onto the image sensor. In order to correct aberrations of the optical system, particularly curvature of field, the image sensor is curved into a spherical shape having a radius of curvature equal to that of the curvature of field. A problem with this arrangement is that such an image sensor is very difficult to manufacture. A further problem is that it is difficult to accommodate different image sizes in this type of image sensor.
In U.S. Pat. No. 4,843,481, there is disclosed CCD scanning apparatus for use with a rotary head printer. This apparatus comprises an area image sensor in which the image sensor elements are arranged in rows having a curvature. Data recorded by the image sensor is printed on a rotary head printer which produces a scan line having the same curvature as the image sensor elements, and thus, the data from the image sensor can be directed to the printer without additional processing. A disadvantage of the scanning apparatus disclosed in this patent is that the optical system used with the scanner would not produce an aberration-free image on the image sensor, and thus, the scanner would not be suitable for applications requiring high resolution images. Further, the area image sensor could not be used for the high-speed scanning of a continuously moving web such as film.
The patent to Offner, U.S. Pat. No. 3,748,015, discloses an optical system for forming an image of an object at unit magnification. The system incorporates a convex mirror and a concave mirror arranged with their centers of curvature coinciding at a point. Such an optical system provides a substantially distortion-free image. However, there is no provision in this patent for digitally recording an image produced by the optical system.